With this kind of image forming apparatus, a heat roller type, belt type, or similar heat-fixing apparatus has traditionally been used as a fixing apparatus that heat-fixes an unfixed toner image formed and held on a recording medium such as plain paper or an OHP sheet.
A known example of this kind of heat-fixing apparatus is provided with a rotatable heating roller that has a heat source such as a halogen lamp or heater, an endless fixing belt that presses against this heating roller and rotates together with this heating roller, and a pressure pad that is positioned on the inner side of this fixing belt, presses the fixing belt against the heating roller, and forms a fixing nip by means of the pressure area between the fixing belt and the heating roller (see, for example, Unexamined Japanese Patent Publication No. HEI 10-228196).
In recent years, due to demands such as shorter warm-up time and better energy-saving characteristics for image forming apparatus, attention has been drawn to heat-fixing apparatuses that use a heat-producing section employing induction heating (IH), which allows rapid and highly efficient heating, as the heat source, and reach the desired image fixing temperature in a short time (See, for example, Unexamined Japanese Patent Publication No. HEI 10-123861).
FIG. 1 is a schematic configuration diagram of a heat-fixing apparatus that uses an IH heat-producing section as the heat source. As shown in FIG. 1, this heat-fixing apparatus has an exciting coil 14 located inside a fixing roller 12 serving as an image heating body, and fixing roller 12 is made to produce heat by causing generation of an alternating field by means of this exciting coil 14 and a core 17 composed of ferrite or the like, and causing an eddy current to be generated in fixing roller 12. A recording medium 10 bearing an unfixed toner image 11 is then fed to the pressure area between fixing roller 12 and a pressure roller 13, and unfixed toner image 11 is fixed onto recording medium 10.
Another heat-fixing apparatus using an IH heat-producing section as a heat source has also been proposed that has a configuration in which a fixing roller serving as an image heating body is formed as a thin metal sleeve, and this metal sleeve is sandwiched and rotated by inner and outer pressure members (see, for example, Unexamined Japanese Patent Publication No. HEI 10-74007).
In a heat-fixing apparatus used in this kind of conventional image forming apparatus, the temperature of the image heating body such as a fixing belt, fixing roller, and so forth is normally measured by means of a temperature sensor positioned in contact with the image heating body. The calorific value of the heat source is then controlled based on the measurement result of this temperature sensor so that the temperature of the image heating body is maintained at an image fixing temperature suitable for heat-fixing an unfixed toner image onto a recording medium.
In the case of a heat-fixing apparatus that uses a halogen lamp or heater as a heat source, the warm-up time until the image heating body rises to the predetermined image fixing temperature is long, and therefore the thermal time constant of the temperature sensor has not been a problem.
However, in the case of a heat-fixing apparatus that uses an IH heat-producing section as a heat source, or that uses a fixing belt as an image heating body, the warm-up time is short (30 seconds or less, for example), and therefore the thermal time constant of the temperature sensor has an effect.
That is to say, while the use of an inexpensive temperature sensor is generally desirable with this kind of image heating apparatus in order to keep costs low, there is a drawback that an inexpensive temperature sensor normally has a large thermal time constant, and responsiveness to a rapid change in temperature is poor. Therefore, if a temperature sensor with a large thermal time constant is used in an image forming apparatus that has a short warm-up time, there is a problem of the actual temperature of the image heating body rising before the temperature detected by the temperature sensor rises, resulting in major overshoot.
Also, in this kind of image forming apparatus, control is performed so that an image forming operation is started when the temperature detected by the temperature sensor reaches the predetermined image fixing temperature. Therefore, if a temperature difference occurs between the actual temperature of the image heating body and the temperature detected by the temperature sensor as described above, there is a problem in that the first print start time is delayed in proportion to this temperature difference. This kind of first print start time delay is pronounced when image forming is performed after the image heating body is heated from a state in which the environmental temperature of the heat-fixing apparatus is close to room temperature.
Thus, a problem with this kind of conventional image forming apparatus is that the actual temperature of the image heating body becomes higher than the predetermined image fixing temperature due to the first print start time delay, and the glossiness of the first printed sheet is abnormally high.
The occurrence of print defects due to such first print start time delay can be eliminated by using a temperature sensor that is highly responsive to rapid temperature changes and has a small thermal time constant. However, a problem associated with the use of a temperature sensor with a small thermal time constant is increased cost.